1. Field of the Invention
The subject invention relates to multilayer prepregs suitable for preparing extreme damage tolerant fiber reinforced composites. More particularly, the subject invention relates to multilayer prepregs prepared by laminating thin films of particular engineering thermoplastics with a layer of reinforcing fibers impregnated with a thermosetting resin containing substantial quantities of one or more heat-curable cyanate-functional resins. These prepregs may be stacked and cured to form fiber reinforced composites which exhibit extreme damage tolerance.
2. Description of the Related Art
The use of fiber reinforced composites has grown rapidly, especially in the transportation and aerospace industries. It is well known that composites containing fiber reinforcement such as high strength organic fibers, glass fibers, and particularly carbon/graphite (C/G) fibers can replace most structural metals in many applications. Such replacement frequently results in simultaneously increasing structural strength while decreasing mass. Such applications are particularly well suited to aerospace applications where high strength to weight ratios are desirable.
Among the fibrous reinforcing materials most frequently used in these applications are the carbon/graphite fibers. Prepregs containing C/G fibers may be readily fabricated using conventional techniques. For example, woven and non-woven C/G mats or fabrics, or colminated, unidirectional fiber tows may be impregnated with a heat curable thermosetting resin. The impregnating resin may be supplied in the form of a solution in a suitable non-reactive solvent, neat resin in the melt, or preferably neat resin supplied as one or more thin films.
The melt impregnation of fiber reinforcement with such films is well described in U.S. Pat. No. 3,784,433, for example. In this method, the fiber reinforcement is fed between heated rollers with one or more films of thermosetting resin. The combination of heat and pressure forces the resin into the fibers. Subsequent travel through additional rollers, if necessary, serves to work the resin into the fibers, assuring good fiber to-resin contact, and therefore, prepreg uniformity. During this process, the temperature of the resin film and prepreg are carefully controlled. Generally, the resin partially cures, or "advances" during this process to the still fusible "B-stage." If the temperature is too high, or the time the prepreg is exposed to processing temperatures too long, the resin may advance too far and the prepreg will not be suitable for its intended use in preparing composites. Adjustment of process variables is routinely and successfully practiced by those skilled in the art.
The finished prepregs are generally stored at low temperatures until just before use. Composites are produced from the prepregs by stacking the desired number of prepreg plies together, and curing under heat and pressure. During cure, the B-staged resin at first melts and flows, and then cures to the infusible "C-stage." Following cooling, the resulting composite has a strong, integral structure.
Although the composites produced by the above process are generally far stronger per unit weight than the common structural metals such as steel, aluminum, titanium and magnesium, such composites suffer from several disadvantages. Because of the chemical makeup of the resins utilized in the composites, strength decreases rapidly as the temperature increases. Furthermore, the resins are susceptible to damage by organic solvents and are generally combustible. Although these shortcomings limit somewhat the application of C/G composites, a more serious limitation is the lack of resistance to impact damage.
While C/G composites are exceptionally strong, their "toughness" is limited. When subjected to moderate to severe impact, extensive local damage may occur, resulting in a severe loss of overall strength in the composite. The damage caused by sudden impact is measured qualitatively by assessing the size of the damage area through application of ultrasound scanning techniques; or quantitatively by measuring compression after impact (CAI) values. For example, a 1500 in-lb/in impact by a 10.0 to 12.0 lb test weight having a 0.5 in hemispherical impact surface will frequently produce a damage area exceeding 4 in.sup.2. The loss in compression strength may at times be in excess of 50 percent and is typically between 25 and 40 percent.
The large impact damage area and the loss of compressive strength has forced designers to increase the thickness of the composites used in various structures. However, the increase in thickness decreases the weight advantage of composites as compared to other materials. Furthermore, thicker laminates, while stronger, are more prone to impact induced delamination, and thus fairly large increases in thickness produce only modest increases in CAI values.
Attempts to increase toughness in C/G composites has met with but limited success. In U.S. Pat. No. 3,755,059, for example, the addition of a combination of corrugated metal foil and glass fiber reinforcement to C/G composites is suggested as a means of improving impact strength. However, only modest increases are possible with this method. Furthermore, the presence of metal within the composite causes heat distortion problems due to the different coefficients of thermal expansion of the various materials.
In U.S. Pat. No. 3,784,433, the use of plastic and metal foils or films is advocated, together with the use of thermosetting epoxy matrix resins containing significant amounts of polysulfone thermoplastic. However, no examples of the use of such films or foils are given. In U.S. Pat. No. 4,274,901, the use of mylar films is disclosed, however, it was found necessary to perforate the mylar thermoplastic films due to their poor adhesion to the epoxy matrix resin. Furthermore, the decrease in impact damage was achieved by sacrificing the interlaminar shear strength, an unacceptable compromise in advanced structural materials.